1. Technical Field
This invention relates generally to methods for forming tenacious bonds between inserts and cast metal materials such as reinforcing inserts for castings of internal combustion engine components.
2. Description of the Prior Art
The substitution of light weight casting material, such as aluminum alloys, for cast iron is a known approach for weight reduction in a variety of applications such as internal combustion engine manufacture. For example, the use of aluminum alloys to form internal combustion components is well known for automotive engines or high performance racing or aircraft engines. Such substitutions, however, have often resulted in compromised performance and/or reliability.
A well known solution to some of the performance and reliability problems associated with the use of light weight casting material as a substitute for cast iron has been to provide high strength inserts at critical points where severe wear or high stress is known to occur.
One approach has been to substitute aluminum pistons with nickel-iron ring carriers for conventional cast iron designed as disclosed by E Mahle, xe2x80x9cAlloy Iron Ring Carriers Reduce Cylinder Wearxe2x80x9d, Auto Industries, Vol. 68, No. 19, May 1933, p578-82. However, wear of ring lands and rings in aluminum pistons causes reduction in engine output, blow-by of combustion gases, increased oil consumption, increased fuel consumption and piston slap (noise). Early attempts to correct these drawbacks involved placing gray iron inserts which had a coefficient of thermal expansion (CTE) of 0.0000067 in/in-xc2x0 F. in aluminum pistons, which had a coefficient of thermal expansion of 0.0000134 in/in-xc2x0 F. The differences in thermal expansion caused the carrier to loosen. The first successful pistons employed an aluminum-silicon alloy with a CTE of 0.000010 in/in-xc2x0 F. with a Ni-resist carrier of about the same CTE. The Ni-Resist is an alloyed iron with 15% Nickel and 5% copper. When a Ni-Resist with even higher nickel and molybdenum content was used, an aluminum-copper alloy could be used for the piston, with improvements in thermal conductivity, machinability as well as thermal fatigue resistance.
However, when a ferrous alloy of low coefficient of thermal expansion is used, the tendency will be for the aluminum to grow against the restraining iron band during each heating cycle. This may minutely upset the aluminum so that when it cools, the fit will not be as close as it was originally. Subsequent heating cycles will exaggerate this condition until the ring band itself loosens on the piston.
Weight reduction by replacement of cast iron by light weight alloys incorporating inserts has not found general acceptance in heavy-duty diesel engines because of the severe performance and durability demands of the markets in which they have traditionally been used. One explanation for this is the difficulty of achieving an effective, durable metallurgical bond between the insert and the adjacent light weight cast material. For example, in xe2x80x9cEngineering for Aluminum-Alloy Castingsxe2x80x9d, by T R Gauthier and H J Rowe of ALCOA, Mechanical Engineering, Vol. 70, No. 6, June 1948, p505-14, the authors discuss casting design from the standpoint of mechanical properties, section thickness, and the use of inserts. The authors assert that no metallurgical bond normally exists between inserts and aluminum. Projecting legs or dogs or at least knurling are said normally to be necessary to mechanically retain the insert in the casting, especially if a torque will be applied to the insert.
An exception to the general rule that light weight alloys with strengthening inserts are not generally used in diesel engines has been the use of aluminum pistons joined with cast iron ring carriers by an Al-Fin process disclosed in U.S. Pat. No. 2,396,730 to Whitfield et al. Other techniques for pre-coating an insert prior to casting are disclosed in U.S. Pat. No. 2,849,790 to Zwicker et al.; U.S. Pat. No. 2,881,491 to Jominy et al.; U.S. Pat. No. 3,945,423 to Hannig, U.S. Pat. No. 4,997,024 to Cole et al. and U.S. Pat. No. 5,333,669 to Jorstad.
However, debond problems with the Ni-resist iron insert has given the Al-Fin process, and other pre-coat processes, a poor reputation. These pistons suffer from two problems. The first is that during casting the aluminum contracts more than the iron, which may place the interface in tension. The second problem with pistons of this type is the presence of brittle inter metallic compounds. It has been established that cracking in pistons occurs between gamma-Al3FeSi and a Fe3(Si0.9Al.1) phase. The presence of these compounds is a function of casting temperature, cooling rate and bath composition and is not an inherent feature of Al-fin bonding.
J A. Lucas, xe2x80x9cAluminum Cylinder Blocks Cast in Permanent Moldsxe2x80x9d, Am. Mach., Vol., 66, No. 4, Jan. 1928, p173-174, describes a composite engine block including cast aluminum with several inserts. The liners were of cast iron with nickel added for wear resistance and to control thermal expansion. The liners were sand blasted and copper plated prior to being placed in the mold. The best casting alloy with regards to bonding to the inserts, soundness and proper shrinkage was experimentally found to be 99% aluminum 1% copper. Even small percentages of impurities were found to dramatically increase scrap rates.
J H Beile and C H Lund, xe2x80x9cCurrent Status of Composite Casting as Bonding Techniquexe2x80x9d, Metals Eng. Quarterly, Vol. 6, No. 1, Feb. 66, p63-4, disclose a bonding technique for achieving metallurgical bonding requiring an absolutely clean surface on the inserts. Practical methods to prevent oxidation are to employ vacuum, inert atmospheres, or reducing atmospheres.
xe2x80x9cBonding Iron to Aluminum by Casting-Onxe2x80x9d, Light Metals, Vol. 21, No. 248, Nov. 1958, p355-6, describes the principles for producing metallurgical bonds between inserts and casting. This paper reports that the production of an intimate junction may be prevented by the presence of an oxide film on the outer surface of the aluminized coating on the insert.
Another approach that has been undertaken in an attempt to achieve acceptable metallurgical bonding between inserts and cast metal is disclosed in U.S. Pat. No. 5,429,173 to Wang et al. which discloses a casting process wherein the insert, such as a ferrous metal cylinder liner, is pre-coated with plural layers of alternating materials that are exothermically reactive to produce inter-metallic phases at the surface.
Yet another approach toward achieving an acceptably strong bond between cast aluminum alloy, such as would be suitable in forming an engine block, and inserts such as cast iron cylinder liners involves pre-coating the liner with a metal coating. Examples of such approaches are disclosed in U.S. Pat. No. 1,710,136 to Angel et al., U.S. Pat. No. 3,165,983 to Thomas and U.S. Pat. No. 5,005,469 to Ohta. While claims have been made that these methods produce metallurgical bonds, others have reported that such bonds are not formed on a consistent and reliable basis. See for example U.S. Pat. No. 5,280,820 to Voss et. al. This later patent discloses a method which attempts to overcome the deficiencies of the prior art by coating the insert with zinc, tin or cadmium or their alloys in a way to cause an outer oxidized surface to form, followed by the step of removing the oxidized surface, and casting molten metal, such as aluminum based material, around the insert to cause the coating to remelt and alloy with both the insert material and the cast material to form a metallurgical bond between the liner and cast material. While effective for the purposes disclosed, this later approach has the effect of allowing direct contact between the base material of the insert and the molten casting material. This direct contact can give rise to undesirable inter-metallic phases that negatively impact the quality of the bond.
While relevant, none of these prior art methods has been entirely successful in producing consistent, high strength bonds between inserts and light weight casting material that will meet the long term demands for reliability required in certain applications such as the manufacture of heavy duty diesel engine components. For example, prior art processes are prone to producing defective products caused by voids, gas porosity, and oxides. In many cases, the insert will simply drop off from the casting as the number of defects is so great that no metallurgical bond is formed whatever. Therefore weight reduction through the broad application of light weight casting material remains an unmet, yet highly desirable, goal for many applications including diesel engines particularly in certain diesel engine applications such as full size pick-up automotive markets, marine markets and certain military applications.
An important objective of this invention is to overcome the deficiencies of the prior art by providing a casting method that forms high strength joints between inserts and light-metal casting material and by providing castings that consistently achieve an extremely low defect rate, high strength and long term durability.
An important objective of this invention is to provide a casting method for forming highly tenacious bonds such as metallurgical bonds between inserts and the casting material including the step of coating the insert with one or more layers of metallic material to a thickness that allows some, but not necessarily all, of the layer to be dissolved into the casting material to create a diffusion barrier that prevents the formation of undesirable inter-metallics, such as Fexe2x80x94Alxe2x80x94Si.
Yet another objective of this invention is to provide a casting method as described above wherein the step of coating the insert includes coating a first layer onto the insert followed by coating of a second layer followed by a casting step under conditions including sufficient temperature to cause the second layer to be substantially or completely sacrificed by dissolving into the cast metal material while leaving at least a portion of the first layer as a diffusion barrier between the insert and the cast material. The coated layers may be applied by a variety of different coating processes such as electroplating to a thickness of 0.5 to 8 mils but 0.5 to 4 mils is desirable and 1 to 2 mils is still desirable
Another objective is to provide castings formed by the above method that have interfacial bond strengths between the insert(s) and the bonded cast material above 8,000 psi.
Still another objective is to provide castings that can be subjected to a heat treatment process (such as T5 or T6 processes) following casting without degradation of the bond strength or reduction in long term durability of the bond quality between the casting material and the insert.
Another object of the invention is to provide a casting method as described above involving pre-coating of the insert wherein the insert coating(s) has a coefficient of thermal expansion intermediate to that of the coefficients of thermal expansion of the insert material and the casting material.
Another objective of the subject invention is to provide a casting method for forming highly tenacious bonds between inserts and the casting material by seeking to reduce the amount of hydrogen and other contaminates that are absorbed in the molten casting material during melting and mold filling and to reduce otherwise the contamination of the cast material.
Still another object of the subject invention is to provide a casting method in which the molten cast material including aluminum is degassed to reduce the dissolved hydrogen concentration in the melt and thus reduce the amount of hydrogen that will precipitate at the aluminum/insert material interface during solidification.
A still more specific objective of the subject invention is to provide a casting method in which the molten cast material is degassed sufficiently to cause any porosity that may form to be smaller than that which can be seen by the naked eye on a cut transverse section of a reduced pressure test (RPT) sample. This amount of hydrogen should be less than 0.15 parts per million (ppm) and more ideally 0.10 ppm. These levels translate to less than 0.168 cubic centimeters per 100 grams of casting material (cc/100 gms) and 0.112 cc/100 gms.
Still another object of the invention is to provide a casting method for forming highly tenacious bonds between inserts and the casting material including the step of casting within a dry air or dry inert gas environment (such as argon or nitrogen) or under a vacuum to prevent hydrogen from being picked up by the molten casting material during the mold filling process thereby limiting the level of dissolved hydrogen in the molten casting material and keeping the resultant porosity level in the casting low. This objective can be aided by using argon having a moisture content below 3 ppm.
Yet another objective of the invention is to provide a casting method including the provision of a mold that is adapted to receive the insert(s) and that includes one or more inlets through which the molten cast material can enter the mold and one or more outlets through which excess molten casting material can be allowed to overflow during the casting process thereby causing oxides and other contaminates at the leading edge of the metal flow that would otherwise contaminate the interface to flow away from the interface. Alternatively the mold may be designed to direct the flow of the molten casting material into sections of the mold after the molten casting material flows over the coated insert surfaces to allow contaminants to be carried away from the coated surfaces to cause the molten casting material, most likely to be contaminated with oxides and inclusions, to be directed away from the interface between the insert and casting material. This feature of the invention allows the casting to be carried out at lower pouring temperatures since the larger quantity of metal prevents premature freezing of the metal, and thus reduces the dissolved hydrogen concentration in the melt.
It is yet another object of this invention to provide a casting method for forming highly tenacious bonds between inserts and the casting material wherein the casting-insert interface can be made substantially defect free by degassing the molten cast material, casting under dry air or dry inert gas protection or under vacuum and by using a mold that provides for entrained contaminants to flow away from the interface between the insert and casting material.
Yet another object of this invention is to provide a casting method as described above in which the insert may be formed of ferrous material such as carbon steel or stainless steel, the casting material may be formed of light weight metal alloy such as aluminum alloy and still more particularly 354 or A354 aluminum alloy and the coating materials may be selected from a group consisting of Ni, Ag, Cu, Antimony, Bismuth, Chromium, Gold, Lead, Magnesium, Silicon, Tin, Titanium, and Zinc.
Another object of this invention is to provide a casting method as described above including the step of cleaning the coated insert in an alkaline bath followed by the step of the acid cleaning.
Still another object of the invention is to provide a casting method as described above wherein the insert is coated first with Ni and second with Cu and the step of coating the first layer includes coating at an elevated temperature (e.g. approximately 50xc2x0 C.) and the step of coating the second layer includes the step of cooling the coated insert to room temperature and coating the second layer at an elevated temperature (e.g. approximately 40xc2x0 C.). The actual temperatures may vary but it is most important that the second layer be coated at a temperature above room temperature. Since the Cu layer growing on the Ni layer has a higher coefficient of thermal expansion (CTE) than the steel/Ni assembly, it xe2x80x9cshrinksxe2x80x9d on the steel/Ni insert assembly upon cooling. As a result no gap exists between the Ni and Cu layers. After the coatings are applied, the process includes the additional step of annealing the coated insert formed at a temperature of 900xc2x0 C. for example. If the first layer is Cu. the coating temperature may be in the range of 50xc2x0 C. and if the second layer is Ag the coating temperature may be in the range of 40xc2x0 C. and the annealing temperature may be around 720xc2x0 C.
Another object of this invention is to provide a novel casting including pre-coated inserts formed as described above and cast to form components of an internal combustion engine such as the head, block or pistons.
The above objects and advantages of the subject invention may be realized by a casting method including the steps of forming an insert, such as a cylinder liner or head re-inforcing element, formed of carbon steel or stainless steel. The insert surface is prepared and coated by an electroplating process with a first layer of metal, such as Cu, Ni or Ag, having a coefficient of thermal expansion between the coefficients of thermal expansion of the insert material and the casting material, and with a second layer of metal, different from the first, by an electroplating process. The second layer preferably has a coefficient of thermal expansion between that of the first layer and the casting material. In order to insure that no gaps are formed between the layers, the first layer (e.g. Ni) is coated at an elevated temperature (e.g. 50xc2x0 C.), after which the layer is allowed to cool to room temperature. The second layer is coated at an elevated temperature (e.g. 40xc2x0 C.) during which the steel/Ni is heated and expanded. Since the Cu layer growing on the Ni layer has a higher CTE than the steel/Ni assembly, it xe2x80x9cshrinksxe2x80x9d onto the steel/Ni assembly upon cooling. Next the coated insert is annealed at a temperature (e.g. 900xc2x0 C.) sufficient to form appropriate diffusion bonds between the layers and the insert. To commence the casting process, the coated insert is baked at a temperature above 100xc2x0 C. for at least 5 minutes to dispel absorbed moisture. A sand mold is prepared and dried for 6 hours and the insert is placed in the sand mold. The casting material (e.g. A354 or 354 aluminum alloy) is heated to 720xc2x0 C. and the molten casting material is degassed by appropriate techniques such as the use of a rotary degasser or a porous lance. Casting proceeds under an atmosphere of argon to prevent absorption of hydrogen. The sand mold is formed to provide an inlet and an outlet to allow the leading edge of the molten flow to pass as overflow through the outlet thereby carrying contaminants away from the interface between the molten casting material and the coated surface of the insert. Alternatively the mold may be designed to direct the flow of the molten casting material into sections of the mold after the molten casting material flows over the coated insert surfaces to allow contaminants to be carried away from the coated surfaces. Finally the casting may be heat treated via a standardized heat treatment process known as T5 or T6.
Other and more specific objects of this invention may be perceived from the following Brief Description of the Drawings and the Description of the Preferred Embodiment.